or course, but I also want a 80-200 F2.8. Even if I had the money
for a 100-400 L, I would still prefer the 80-200 F2.8 with a 2x tc
because it is more convenient. I got to appreciate the F2.8 for
candid..I really like it. the 100-400 L is nice but can't do F2.8.
The TC solution works really well for what I want.

and I would need a 600mm lens to replace the combo 2x + 1.4x but I
really don't have that kind of money so the tc is all I can afford
for now.

I haven't read the whole thread so pardon my potential redundancy. I have only had passable 1.4X + 2X with one lens ever - the 135 f2.0. Even at that the focus was very slow and the softness was fixable but very much evident. I'm sure that given enough light to stop down to f8 - f11 the results would be better but typically I want this kind of reach for things that also need a fast shutter (sports usually.) If stacking did the job for you with the 300D consider yourself lucky.

It's interesting that you're finding it not quite as useable with the 20D since by 99% of accounts the focus is faster and more accurate with the 20D. One thing I'm curious about: my 10D never "knew" that I had stacked converters - it always detected the stack as if it was just the 2X TC by itself... is this the same with the 20D?